Ink cartridges are used as interchangeable ink tanks for ink supply of inkjet printing devices. Essentially they comprise a housing made for mechanical fixing in the corresponding receiver of a printer and one or more ink containers which can be filled with different inks housed as actual ink reservoirs. Each ink container has an ink outlet for detachable connection to the ink supply connection of the printer. If the ink contained in the ink container has been used up by the printer in operation, the empty ink cartridge can be easily taken out of the printer and replaced by a full ink cartridge. The empty ink cartridges are either thrown away or refilled with ink for re-use.
Smaller inkjet printers of simple structure require ink cartridges in which the ink at the ink outlet is at a given negative pressure level corresponding to the negative pressure produced by the print head during operation at the ink supply connection of the printer. The negative pressure level must be kept within narrow limits: On the one hand, it must be ensured that the ink does not run out by itself due to the force of gravity and disrupt the operation of the print head, on the other hand the pressure level may not be so high that ink can no longer be sucked out by the print head.
It is known to use a differential pressure valve in an ink outlet to keep the ink back in the ink cartridge under negative pressure. It opens when the print head is working and is intaking the ink, by which a negative pressure is produced on the ink supply connection. Then the differential pressure valve opens and ink can flow to the print head.
Ink cartridges with differential pressure valves in the ink outlet are known in diverse versions, for example from EP 238 829 B1. It describes a valve with an elastic valve body sealing the ink outlet channel on the outside and clears it when a predetermined negative pressure level at the ink outlet is not reached by the print head. The use of an elastic valve body does have the advantage of a simple structure. But the disadvantage is that production of an elastic valve body with the required precision of elastic properties in large numbers is relatively complex. Moreover the properties of the elastic materials change with time, so that the valve becomes leaky and the pressure difference required for opening becomes too high or too low.
To some extent, the aforementioned problems can also be equalized by the valve body being loaded with a spring element. This concept is described for example in EP 1 125 747 A2. The use of a metal spring (as shown in FIG. 8) does have the advantage of improved long term stability. In any case, springs with the required precision are complex to produce and are accordingly expensive. Moreover spring installation is complex.
In view of the aforementioned problem in the prior art, the object of the invention is to devise an ink cartridge with a differential pressure valve which is stable over the long term, precise, and easy to produce and install.
To achieve this object, it is proposed as claimed in the invention that a first permanent magnet is attached to the valve body and relative to the valve opening a second permanent magnet is attached securely to the ink cartridge and is opposite the first permanent magnet with the same pole, so that the repelling force between the first and second permanent magnet acts as a closing force on the valve body.